Game Development Reference
In-Depth Information
Quantity
Units
SI unit
Rough translation
Radiant energy
Energy
J
Total illumination during
an interval of time
Radiant flux
Power
W
Brightness of a finite area
from all directions
W/m 2
Radiant flux density
Power per unit area
Brightness of a single point
from all directions
W/m 2
Irradiance
Power per unit area
Radiant flux density of
incident light
W/m 2
Radiant exitance
Power per unit area
Radiant flux density of
emitted light
W/m 2
Radiosity
Power per unit area
Radiant flux density of
emitted or reflected light
W/(m 2 sr)
Radiance
Power per unit
projected area, per
unit solid angle
Brightness of a ray
Table 10.1. Common radiometric terms
a similar term from photometry (Table 10.2) . The only real difference is a
nonlinear conversion from raw energy to perceived brightness.
Throughout the remainder of this chapter, we try to use the proper ra-
diometric units when possible. However, the practical realities of graphics
make using proper units confusing, for two particular reasons. It is com-
mon in graphics to need to take some integral over a “signal”—for example,
the color of some surface. In practice we cannot do the integral analyti-
cally, and so we must integrate numerically, which boils down to taking a
weighted average of many samples. Although mathematically we are taking
a weighted average (which ordinarily would not cause the units to change),
in fact what we are doing is integrating, and that means each sample is
really being multiplied by some differential quantity, such as a differential
area or differential solid angle, which causes the physical units to change.
A second cause of confusion is that, although many signals have a finite
nonzero domain in the real world, they are represented in a computer by
signals that are nonzero at a single point. (Mathematically, we say that
Radiometric term
Photometric term
SI Photometric unit
Radiant energy
Luminous energy
talbot, or lumen second ( lm s )
Radiant flux
Luminous flux, luminous power
lumen ( lm )
lux ( lx = lm/m 2 )
Irradiance
Illuminance
lux ( lx = lm/m 2 )
Radiant exitance
Luminous emittance
lm/(m 2 sr)
Radiance
Luminance
Table 10.2. Units of measurement from radiometry and photometry
 
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